One might also consider that not having registered player names might be a problem for the goal of attracting new players. If a gamer see they can't register their nick and realize anyone can play with their nick, they may decide it's not worth it. Kind of like.. for example, would I want to be a regular user on an IRC network, or a message forum, where I know anyone could log in with my nick anytime they wanted?
Though even a registered user (if registration were required or implemented in the future) could change his nick if he was banned, and be able to re-register with a different IP, and so I acknowledge banning by nicks is not a guarantee; however, if a user loses his stats because his nick was banned, it would discourage him from either re-registering, or discourage him from repeating abusive behavior. Also, some people's IPs don't change very frequently, so banning a registered user may be very effective in many cases.
I don't think there's any immediate rush to implement anything to address any current abusive behavior. I haven't heard from other players that it's a big problem right now, and I've only experienced it... maybe 2x in the last 3 months.
To ensure fairness for players, definitely a set policy needs to be written so players have a clear idea of what's allowed and what isn't. And moderators need to be able to warn players by directing them to read the policy.
As for specific policy content, perhaps a simple derivative of something other game maintainers (or forum community maintainers) already have would be all that's needed.